Male sex is a powerful risk factor for atherosclerotic coronary artery disease (CAD). Various explanations for this relationship exist, but conventional wisdom maintains that testosterone (T) directly or indirectly increases CAD risk. Recent studies, however, have questioned this conclusion and prompted some experts to suggest that high normal T levels may reduce CAD risk. We speculate that T may exert a beneficial effect on vascular function in men after conversion to estrogen. The present pilot study will test the hypotheses that exogenous T improves brachial artery endothelium dependent vascular dilatation in healthy men greater than 65 years of age and that the improvement correlates with the change in estradiol levels. Subjects (N=38) will be randomly assigned to placebo or to 200 mg of intramuscular T enanthate every other week. Arterial endothelial function will be examined at baseline and after 10 weeks of T treatment by measuring the brachial artery response to ischemic stress. The brachial artery response to ischemic- induced, increased flow is an established surrogate for coronary endothelial function. The results will provide insight into how T affects endothelial function and should have direct application to the risk benefit analysis of using T supplementation to prevent frailty in healthy aging men.